A fuel injection arrangement of the above type is disclosed in U.S. Pat. No. 4,700,668. The pulse chamber is charged directly with the pressure present in the crankcase while the return chamber communicates with the atmosphere. The pump piston is moved up and down in correspondence to the pressure variations in the crankcase and injects fuel into the combustion chamber of the two-stroke engine.
The pressure in the crankcase is dependent upon the rotational speed and load of the two-stroke engine. An overpressure develops with a downward movement of the piston in the direction toward bottom dead center; whereas, the pressure in the crankcase drops to an underpressure with the following upward movement of the piston toward top dead center. The crankcase pressure then swings between positive and negative values with the positive values likewise increasing to a maximum with increasing speed which then remain constant up to the highest speed. The pressure oscillations lie, for example, between approximately 0.75 bar and -0.2 bar.
In U.S. Pat. No. 4,700,668, it is suggested that a controllable pilot valve arranged between the crankcase and the pulse chamber be provided for adapting the beginning of the injection in dependence upon the rotational speed by utilizing the pressure conditions in the crankcase. At high rpm, a later injection point is obtained whereas at lower rpm, an earlier injection point is obtained.
By changing the time point of injection in this manner, the quantity of injected fuel however remains substantially unchanged so that the mixture becomes too rich at high rpm because of the changed air charge of the cylinder. In order to counter the formation of an overrich mixture, a throttle is placed in the connecting line from the crankcase to the pulse chamber which is effective at high rpm. This measure by itself is not adequate to adapt the quantity of fuel with sufficient precision to the rpm.